1. Field of the Invention
The present invention relates to a duobinary optical transmission device using a duobinary optical transmission technique. More particularly, the present invention relates to a duobinary optical transmission device using at least one semiconductor optical amplifier (SOA).
2. Description of the Related Art
An optical transmission system based on dense wavelength division multiplexing (DWDM) transmits an optical signal comprising a plurality of channels having different wavelengths that are transmitted on a single optical fiber, thereby increasing the transmission efficiency. Due to the fact that the optical transmission system can transmit the optical signal irrespective of a transmission speed, there has been a recent increase in the use of very high-speed Internet networks, and such networks have recently increased transmission capacity in order to meet demand. Already, there is a known system that transmits at least 100 channels through a single optical fiber using the DWDM which has been commercialized. Research on a system having a transmission speed of 10 Tbps or more for simultaneously transmitting at least 200 40-Gbps channels to the single optical fiber is being actively conducted.
However, the extension of transmission capacity in an optical system is limited due to interference and distortion between channels at a channel interval of 50 GHz or less at the time of performing optical intensity modulation using existing non-return-to-zero (NRZ) protocol. The interference and distortion is related both to increased data traffic and high-speed data transmission demands of 40 Gbps or more. When a direct current (DC) frequency component of an existing binary NRZ transmission signal and a high-frequency component transmitted by spread spectrum modulation are propagated in an optical fiber medium, problems associated with non-linearity and dispersion are sufficient so as to limit the transmission distance is limited in a high-speed transmission of 10 Gbps or more.
Recently, optical duobinary technology has been attracting interest because many in the field believe that duobinary technology is the one optical transmission technology that may be capable of overcoming the limitations of transmission distances due on account of chromatic dispersion. An important attribute of duobinary transmission is that there is a reduced transmission spectrum in comparison with other conventional optical transmission schemes. In a dispersion-limited system, a transfer distance is inversely proportional to a square of a transmission spectrum bandwidth value. This means that the transfer distance increases four times when a transmission spectrum is reduced by ½. In addition, as a carrier frequency is suppressed within a duobinary transmission spectrum, the limitation of output optical power due to Brillouin scattering stimulated within an optical fiber can be mitigated.
FIG. 1 is a block diagram illustrating one configuration of a conventional duobinary optical transmission device; and FIGS. 2A to 2C are eye (wave) diagrams illustrating output signals of nodes A, B and C shown in FIG. 1. The conventional duobinary optical transmission device will now be described with reference to FIGS. 1 to 2C.
In FIG. 1, the conventional duobinary optical transmission device includes a pulse pattern generator (PPG) 10 for generating a non-return-to-zero (NRZ) electrical pulse signal based on two levels; a precoder 20 for encoding the 2-level NRZ electrical signal; low pass filters (LPFs) 30 and 31 for converting 2-level NRZ electrical signals outputted from the precoder 20 into 3-level electrical signals and reducing bandwidths of the signals; modulator drive amplifiers 40 and 41 for amplifying the 3-level electrical signals and outputting optical modulator drive signals; a laser source or laser diode (LD) 50 for outputting a carrier wave; and a Mach-Zehnder interferometer type optical intensity modulator 60.
The 2-level pulse signal generated by the PPG 10 is encoded by the precoder 20. An output eye diagram at the node A is shown in FIG. 2A. Furthermore, 2-level binary signals outputted from the precoder 20 are input into the LPFs 30 and 31, respectively. The LPFs 30 and 31 have a bandwidth corresponding to approximately ¼ of a clock frequency of the 2-level binary signal, respectively. Interference between codes due to an excessive limit of the bandwidth occurs, and the 2-level binary signals are converted into 3-level duobinary signals because of the interference between codes. An output eye diagram at the node B is shown in FIG. 2B. The 3-level duobinary signals are amplified by the modulator drive amplifiers 40 and 41, and the amplified 3-level duobinary signals are used as signals for driving the Mach-Zehnder interferometer type optical intensity modulator 60. A phase and light intensity of the carrier wave output from the laser source 50 are modulated according to the drive signals input into the Mach-Zehnder interferometer type optical intensity modulator 60, such that an optical duobinary signal based on two levels is outputted. An output eye diagram at the node C is shown in FIG. 2C. In FIG. 1, “ Q” denotes an inversion signal of “Q”. The 3-level duobinary signals are inputted into the Mach-Zehnder interferometer type optical intensity modulator 60 through the LPFs and the drive amplifiers, respectively.
The Mach-Zehnder interferometer type optical intensity modulator is often based on a Z-cut structure and an X-cut structure according to an arm structure. As shown in FIG. 1, the Mach-Zehnder interferometer type optical intensity modulator based on the Z-cut structure having dual arms is coupled to the electrical LPFs 30 and 31 and the modulator drive amplifiers 40 and 41 at both the arms so that the 3-level electrical signals can be applied to both the arms. The Mach-Zehnder interferometer type optical intensity modulator based on the X-cut structure having a single arm (not shown) is coupled to an electrical LPF and a modulator drive amplifier at the single arm so that a 3-level signal can be applied to the single arm.
However, the conventional structure is significantly affected by a pseudo-random bit sequence (PRBS) because the 3-level electrical signal is output by the electrical LPF therein. As the length of the PRBS increases, transmission characteristics are further degraded, such that it is difficult for the system to be implemented. In particular, a system margin is significantly reduced in case of a 231−1 PRBS rather than a 27−1 PRBS. Conventionally, a slope in the case where an applied NRZ signal is changed from a “0” level to a “1” level is different from that in the case where the applied NRZ signal is changed from the “1” level to the “0” level. However, there is a structural problem in that a shift from the “0” level to the “1” level, or a shift from the “1” level to the “0” level occurs and hence the jitter of an output waveform increases, because parts having differing slopes are combined in case of a duobinary optical transmitter using the electrical LPF. This problem occurs in the conventional Z-cut or X-cut structure. The dependence of this signal pattern causes the system margin to be limited when optical transmission is performed.
In order for the above-described problem to be addressed, a structure in which no electrical LPF is used has been proposed. FIG. 3 shows an example of another configuration of a conventional duobinary optical transmitter using a phase modulator and an optical band pass filter (OBPF). The conventional duobinary optical transmitter shown in FIG. 3 includes a pulse pattern generator (PPG) 10, a precoder 20, a modulator drive amplifier 40 and a laser source or laser diode (LD) 50, similar to that which is shown in FIG. 1. However, the conventional duobinary optical transmitter shown in FIG. 3 uses no electrical LPF, but does include a phase modulator 70 and an optical band pass filter OBPF 80. Thus, the conventional duobinary optical transmitter in FIG. 3 is capable of generating signal characteristics that are similar to the characteristics of the duobinary optical output in FIG. 1.
Another conventional technique that can ensure a constant transmission quality is a technique that transmits according to the length of a pseudo-random bit sequence (PRBS). However, there are problems in this conventional technique in that an expensive phase modulator must be used, which does not permit implementation of a cost-effective transmission device.